1. Field of the Invention
The invention relates in general to implantable medical devices, and specifically to a process for fabricating soft tissue elastomeric implants having surface roughness.
2. Description of the Related Art
Current medical technology has recognized the benefits of a textured surface morphology on medical implants in producing non-classical tissue response to medical implants in the soft tissue (fascia, muscle, adipose, etc.) of a living body, such as a mammal, animal or human. The introduction of a foreign object into soft tissue produces a "classical" tissue response wherein a relatively thick, dense fibrous capsule or a layer of highly oriented collagen forms around the object. This fibrous encapsulation of implantable medical devices or surgical implants, such as prostheses, tissue expanders, breast or mammary prostheses or implants, mass transport or drug infusion devices, and sensors, leads to a common problem referred to as "capsule contracture." In capsule contracture, the fibrous capsule, over time, contracts and squeezes the implanted medical device. If the implanted device is soft and pliable, such as a human mammary implant, contracture causes the implant to deform and tighten, losing its natural shape and softness. Moreover, capsule contracture may also adversely affect the performance of an implanted mass transport device or sensor by inhibiting the diffusion of a therapeutic substance from the mass transport device or by inhibiting the flow of bodily fluid to the sensors. A textured surface with an array of small, closely spaced projections has been proven to reduce the amount of scar tissue, enhance the vascularization near an implant surface, interfere with long-range ordering of collagen, and improve the anchorage for the implant.
Various methods of producing textured surfaces with projections have been proposed. U.S. Pat. No. 4,955,909 to Ersek et al. discloses the texturing of an uncured silicone surface by thrusting selected molecules of a sufficient size and shape onto the surface with sufficient impact to alter the surface morphology prior to vulcanization of the silicone. U.S. Pat. No. 4,960,425 to Yan et al. discloses a similar procedure wherein textured screens with particular patterns are pressed upon the unvulcanized or partially vulcanized silicone shell on the implant before the silicone shell is cured. The use of textured screens with round perforations creates generally columnar projections. The use of textured screens with a crossed or knitted pattern creates projections or indentations of a regular geometric shape. In particular, Yan et al. teaches a preferred texturizing medium to be a reticulated foam which yields projections having irregular shapes when pressed upon the uncured silicone shell. U.S. Pat. No. 5,263,986 to Noiles et al. discloses the use of sintering or plasma spraying of particles onto the implant surface to create a rough surface morphology. U.S. Pat. No. 3,905,047 to Long discloses the use of selective surface etching, as by acid or the like, to create a rough surface texture. U.S. Pat. No. 5,227,602 to Kuhn teaches the use of a spark erosion process to form a dental prosthesis from a solid matrix body.
In summary, processes exist which alter the surface morphology of an implant by physically indenting the surface of an unvulcanized or partially vulcanized silicone outer shell of an implant prior to curing the silicone. Other processes include sintering particles to the surface; subjecting the surface to corrosive chemicals; and using electric sparks to form the prosthesis. Other physical techniques, well known in the industry, include laser etching, scratching, carving, burning, or the like, to cause a sufficient texturization of the surface.
The use of molds is also well known in the industry. Yan et al. teaches the use of injection molding to cast a textured silicone shell by using a textured mold. However, Yan et al. does not disclose how the mold was textured. U.S. Pat. No. 5,002,572 to Picha discloses the use of molding procedures to form micropillars on the implant surface. Picha further discloses that the mold may be textured by means of laser etching techniques, ion milling, or chemical etching, all well known in the art. Other methods well known in the art for texturing the mold include manual indentation of the surface of the mold and sputter texturing of fluoropolymer molds. However, these methods of texturing the mold require extensive manual efforts and are costly.